1. Field of the Invention:
The present invention relates to a bipolar transistor, and more particularly to a structure for improving the gain and the switching speed of the bipolar transistor.
2. Description of the Prior Art:
The bipolar transistor has a higher operating speed than the field effect transistor and, therefore, has been widely used in computers and so on. The bipolar transistor, however, has a limited switching speed, due to carriers accumulated in the collector region. That is, when the conducting operation is saturated, the base-collector junction is forward biased to allow the injection of minority carriers into the collector region, resulting in an accumulation of carriers in the collector region. Once saturated, the transistor does not turn off until the accumulated carriers are drained. This is a cause of the limited switching speed. In order to prevent the transistor from being saturated, an ECL circuit uses a high power voltage. However, the transistor structure used in the ECL circuit is so large that it may withstand the high power operation. Such large structure is a bar for a large integration of integrated circuit.
As another structure for preventing the transistor from the charge accumulation in collector region, a hetero-junction was used for the collector junction as disclosed in [PROCEEDINGS OF THE IEEE] Vol. 70, No. 1 (January 1982), pages 64 to 76. The collector of the proposed structure was made of a semiconductor material having conduction and valence bands positioned outside the band gap of the semiconductor material of the base. According to such structure, when the transistor is saturated, there is generated a barrier between base and collector against the minority carriers in the collector. Therefore, the minority carriers are not injected from the base and are not accumulated in the collector. However, the minority carriers in the base are injected from the collector to the base. Due to the accumulation of the charges of minority carriers in the base, the switching speed of the transistor is still slow. Furthermore, when the base-collector junction is backward biased for normal operation, there is generated also a barrier for majority carriers in the collector which are injected from the emitter through the base, resulting in a small gain. The small gain is improved by grading the energy band between the base and the collector. But such band grading weakens the effect of preventing the minority carrier accumulation.